One kind of anti-skid control device is disclosed in U.S. Pat. No. 4,912,641. This device includes a wheel brake for applying a braking force to a wheel of a vehicle, a master cylinder for generating brake fluid pressure in response to the degree of depression of the brake pedal and for transmitting the generated brake fluid pressure to the wheel brake, a brake fluid pressure control valve interposed between the wheel brake and the master cylinder, a wheel speed detection device for detecting rotational speeds of the wheels, a start determining mechanism for determining whether or not it is necessary to start antiskid control in response to an output signal from the wheel speed detection device, and a pressure reduction determining device for determining whether or not it is necessary to reduce the brake fluid pressure applied to the wheel brake after the antiskid control start determining mechanism has determined that it is necessary to start the antiskid control.
This device further includes a brake fluid pressure control device which operates the brake fluid pressure control valve so that the brake fluid pressure in the wheel brake is reduced if the pressure reduction determining device has determined that it is necessary to reduce the pressure and operates the pressure control valve so that the duty brake fluid pressure in the wheel brake is increased periodically if the pressure reduction determining device has determined that it is not necessary to reduce the pressure. Also, the brake fluid pressure control device measures the period or duration of the duty pressure increase control so that the initial value of the duty pressure increase during the duty pressure increase control following the pressure reduction subsequently performed can be set to a large value if the period or the duration of the duty pressure increase control is longer than a reference value and can be set to a small value if the period or the duration is shorter.
This device also includes a hydraulic pump designed to operate and draw the brake fluid pressure from the wheel brake during the antiskid control to decrease the brake fluid pressure in the wheel brake, and a termination determining mechanism for determining that it is necessary to terminate the antiskid control when the duration of the duty pressure increase control is found equal to or larger than a predetermined duration. Furthermore, in this device, the operation of the hydraulic pump is stopped after it is determined that the antiskid control should be terminated by the termination determining device, that is when the antiskid control is terminated.
However, according to this device, the operation of the hydraulic pump is stopped when the antiskid control is terminated. That is, the hydraulic pump operates as long as the antiskid control continues so that the brake fluid pressure will not reach the level of the hydraulic pressure generated by the master cylinder (hereinafter referred to as M/C pressure) even when the duty pressure increase control is started.
Furthermore, it is determined that the antiskid control should be terminated when the duration of the duty pressure increase control is found equal to or longer than the predetermined duration. Therefore, even when the M/C pressure is close to a wheel locking pressure and the locking pressure is larger than a maximum brake fluid pressure of the wheel brake during the operation of the hydraulic pump, that is including when the control should not be terminated, the control will always be terminated when the duration of the duty pressure increase control exceeds the predetermined duration. More particularly, there is a possibility that the control will be terminated even when the control should not be terminated, which decreases the accuracy in determining whether or not such termination is necessary. Once the control is erroneously terminated, the control will have to be started and terminated repeatedly.
According to the device described in the foregoing, in order to prevent a pressure increase gradient from becoming too steep during the duty pressure increase control, the initial value for the duty pressure increase in the initial duty pressure increase control is set to a relatively small value. Thus, when the M/C pressure (i.e., the degree to which the brake pedal is depressed) is close to the wheel locking pressure (i.e., a small value), the pressure increase gradient during the initial duty pressure increase control becomes too gentle, which causes a delay in the pressure increase and gives rise to a possibility that vehicle deceleration may drop drastically. As described in the foregoing, in the case of the conventional device, unnecessary termination and start of the control has to be repeated, and so the duty pressure increase control has to be carried out based on a relatively small initial value which is reset each time the control is terminated. Consequently, the frequency of a sharp reduction in vehicle deceleration increases.